| 加利福尼亚洋流北区早更新世以来似太平洋年代际振荡的微体化石记录 |
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| 引用本文： | 苏新,陈芳,于翀涵,郭策.加利福尼亚洋流北区早更新世以来似太平洋年代际振荡的微体化石记录[J].地学前缘,2020,27(6):128-143. |
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| 作者姓名： | 苏新 陈芳 于翀涵 郭策 |
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| 作者单位： | 1.中国地质大学(北京) 海洋学院, 北京 1000832.广州海洋地质调查局, 广东 广州 5100753.南方海洋科学与工程广东省实验室(广州), 广东 广州 511458 |
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| 基金项目： | 国家自然科学基金项目(49876015);国家自然科学基金项目(40246002);国家自然科学基金项目(41576062);中国大洋钻探计划委员会基金资助项目;南方海洋科学与工程广东省实验室(广州)人才团队引进重大专项项目(GML2019ZD0209) |
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| 摘 要： | 加利福尼亚洋流系统(CCS)是东北太平洋一股十分重要的边界流,在全球海气交换过程中具有重要地位。CCS的海洋-大气变化尤其是涌升流的强弱变化,在年代和多年代时间尺度上的变化主要受太平洋年代际震荡(PDO,20~30年周期)的控制。为了解CCS北区早更新世以来涌升流强弱变化规律及其主要控制机理,本文分析了国际大洋钻探1020B(CCS北区南部41°N)和1245B(北部44°N)钻孔钙质超微化石和硅藻化石等记录。结果显示,多半时间段的记录具有冰期和间冰期旋回的变化;其中超微化石的高含量见于间冰期,而硅藻高含量对应于冰期。这两类化石在沉积物中的含量变化显示出时间尺度长约0.5 Ma的3个阶段:早更新世(1.5~1.1 Ma)两个孔以超微化石含量高为特征;中更新世(1.1~0.65 Ma)两孔中的超微化石和硅藻化石含量相对较高,但它们在两个孔的变化趋势不同;晚更新世(0.65 Ma)以来,南部1020B孔超微化石含量高且出现频率高于硅藻化石,而北部1245B孔以硅藻化石含量较高且持久出现为特征。本文提出了该区涌升流受长周期“似太平洋年代际振荡模式”和“似北太平洋环流振荡模式”控制的假设。早更新世(1.5~1.1 Ma)主要受到持续的似PDO正位相 (似+PDO,表层海水温暖)的控制,涌升流弱,海水中碳酸盐离子丰富,利于颗石藻骨骼钙化形成颗石和颗石产量高,沉积物中超微化石含量高。中更新世过渡期(MPT,1.1~0.65 Ma)受到似PDO负位相(似-PDO,海水表层水体冷、涌升流强、海水pH值低,硅藻生产力高,沉积物中硅藻化石含量高)与似+PDO的交替影响。晚更新世(0.65 Ma)以来,南部1020站位受到似-PDO/似-NPGO?的交替控制,但似-NPGO?的强度和出现频率均大于似-PDO,因此超微化石含量和出现时期比硅藻多;北部1245站位则仅受似-PDO的持续性控制,涌升流较强且持续时间长、沉积物中硅藻化石几乎持续出现且含量较高。MPT时期以硅藻化石含量在1.1 Ma突然增加(南部增加3倍,北部5倍)为起始点,以0.65 Ma时超微化石含量突变(南部增加3倍,北部降3倍)为终止点。本文认为MPT起止点化石含量的突变,是古海洋和气候环境突变的结果,也意味着长时间尺度的似PDO的正负位相之间的转换变化可能是突变过程。MPT时期CCS北部区域内出现了南部和北部站位硅质和钙质两类化石含量变化趋势相反的记录,指示着这一时期该区的古海洋和气候条件出现了地理上的南北分异,并且自0.65 Ma以来南北两个站位控制模式不同。本文的这些记录为CCS区现代海洋-大气过程和生物地理分区大致以40°N为界的研究提供了早更新世以来的历史记录。
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| 关 键 词： | 钙质超微化石 硅藻化石 涌升流 加利福尼亚洋流系统 太平洋年代际振荡(PDO) 中更新世过渡期 |
| 收稿时间： | 2020-03-19 |
A PDO-like record documented by microfossils from the northern region of the California Current System since the Early Pleistocene |
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| SU Xin,CHEN Fang,YU Chonghan,GUO Ce.A PDO-like record documented by microfossils from the northern region of the California Current System since the Early Pleistocene[J].Earth Science Frontiers,2020,27(6):128-143. |
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| Authors: | SU Xin CHEN Fang YU Chonghan GUO Ce |
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| Institution: | 1. School of Ocean Sciences, China University of Geosciences (Beijing), Beijing 100083, China2. Guangzhou Marine Geological Survey, Guangzhou 510075, China3. Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China |
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| Abstract: | The California Current System (CCS) is a very important boundary current in the Northeast Pacific Ocean and it plays an important role in the global air-ocean interactions. On the interannual and decadal timescales, the ocean-climate change and activity of upwelling in the CCS are principally affected by the Pacific Decadal Oscillation (PDO, with variable periods ranging from 20 to 30 a). To understand the variation of upwelling activity in the northern region of the CCS since the Early Pleistocene and its main control mechanism, we analyzed the data of calcareous nannofossils (fossils of coccolithophores) and diatom fossils from the International Ocean Drilling Program Holes 1020B (southern site, at 41° N) and 1245B (northern site, at 44° N). Our results suggested that abundance variations in these two fossil groups were correlated with most glacial-interglacial cycles, by which high abundances of diatom fossils and nannofosills were associated with glacial and interglacial stages, respectively. Three ca.0.5 Ma long-term stages of fossil abundance variations were recognized: the Early Pleistocene Stage (1.5-1.1 Ma) featured by abundant nannofossils in both holes, the Middle Pleistocene Stage (1.1-0.65 Ma) by relatively abundant diatoms and nannofossils with opposite variation trends in the two holes, and the Late Pleistocene Stage (since 0.65 Ma) by relatively abundant diatoms with very low and sporadic nannofossils in Hole 1245B and contrarily, abundant nannofossils with less abundant diatoms in Hole 1020B. We proposed a mode of long-term dominance of “Pacific Decadal Oscillation (PDO-like)” in combination with “North Pacific Gyre Oscillation (NPGO-like)” mode to explain the possible mechanisms for these records. The northern region of the CCS had been affected by a long-term dominance of +PDO-like phase that led to decreased upwelling during the Early Pleistocene, and by an altering of -PDO (intensified upwelling ) and +PDO phases over the Middle Pleistocene Transition period (MPT). And for the last 0.65 Ma, the region was affected by an altering of strong -NPGO and weak -PDO phases for Hole 1020B, but only by -PDO phase for Hole 1245B. An abrupt increase in diatom abundance (by three times in Hole 1020B and five times in Hole 1245B) marked the setup of MPT at 1.1 Ma, and a significant change in abundances of calcareous nannofossils (3-fold increase and reduction at the southern and northern sites, respectively) indicated the end of MPT at 0.65 Ma. This altering of microfossil abundances was seen as the results of abrupt palaeoceanographic changes. It further implied that the shift from a positive to a negative phase of long-term PDO-like dominance period may be abrupt too. The opposite changing trends of these two fossil groups at the northern and southern sites during and after MPT indicated possible geographical differentiation of ocean-climate conditions in the northern region of the CCS. It provided the Pleistocene records for the study of modern air-ocean processes and biological distribution with the regional boundary at 40° N. |
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| Keywords: | calcareous nannofossils diatom fossils upwelling California Current System PDO-like Middle Pleistocene Transition |
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